The use of short-range miniature radios for communications has increased significantly in the last several years. With the widespread adoption of the Bluetooth standard, the introduction of network protocols like IEEE 802.11x, and the development of single chip radios, the potential for rapid market growth exists in applications ranging from wireless keyboards, Short Range Devices (SRD), and radio sensors for in-vivo medical diagnosis and monitoring.
In the case of in-vivo medical devices, which require 2-way communications, such as hearing aids and implantable sensors, the overall size and power dissipation of the device are of outmost concern. In addition to their communications and sensing circuits, these devices require a discrete and exhaustible power source (typically a battery) to operate. Thus, the operational life of the device is completely dependent on the life of the battery. In some cases, replacement of a discharged battery may even require invasive surgery.
Wireless communications equipment, such as cellular and other wireless telephones, wireless networks (WiLAN) components, mobile radios and other wireless devices are enjoying increasing popularity in the contemporary consumer market. One reason for their increase in popularity is the large number of applications that such devices are now capable of supporting and the wide availability of wireless services. Wireless devices also continue to expand their use in commercial, medical and military applications for example in in-vivo medical applications for similar reasons. As the acceptance and use of wireless devices grows, consumers and commercial users are demanding power efficient communications.
While many radio chips use stand alone transmitters or receivers functionality, most applications require two-way radios. The usual way of adding a receiver to a transmitter is by adding a transmit/receive switch to connect the antenna to either the transmit or receive chain and adding a receiver block. The receiver block typically includes a Low Noise Amplifier (LNA), mixer, IF amplification, and either IF processing for data demodulation or another conversion to baseband. The circuits are quite complicated and require at least doubling of the radio size. Further, these receiver components consume power, thus contributing to shortened battery life.
For purpose of background, it will be appreciated that oscillators are amplifiers with a feedback mechanism. When certain conditions are met (Barkhausen conditions: sufficient open loop gain when the phase is 0 degrees), the device will oscillate. Stated mathematically, if G(s) is the forward gain of the oscillator and H(s) its resonator (feedback device), then the close loop of this system is given (like for all linear feedback systems) by: Z(s)=G(s)/1+G(s)H(s). In practice, the oscillator oscillates at the frequency in which the denominator is 0, or generally, G(s)H(s)=−1.
A significant amount of literature exists in the public domain which describes oscillator theory and non-linear simulation tools used to predict phase noise and other parameters. Generally, as is well known, oscillators generate a narrow band noise signal, and must operate in the non-linear zone of the active device because the open loop gain must typically be at least 6 dB to begin oscillation. When the apparatus settles, it has an average gain of 0 dB (|G(s)H(s)|=1) and hence, inherent non-linearity. In all oscillators, the signal spends a limited time in the linear region before it enters non-linearity of the active device to produce an overall gain of 1. As will be appreciated, oscillators have a high close-loop gain close to the oscillating frequency (denominator goes to zero). This feature, combined with their inherent non-linearity, might result in injection locking, which is regularly observed in the laboratory.